Fix empty sequence adding to embedded bug

[exorevan]

Implement a check to ensure the sequence length is greater than 0 before adding the field to the embedded dictionary.

For example: if field value is '' then it goes to embedded dict from the model

[ELC]

Could you provide a minimal example where this is a problem?

[exorevan]

Of course:

>>> import fastapi_hypermodel
>>> class Process(fastapi_hypermodel.HALHyperModel):
...     number: int
...     message: str | None
... 
>>> process_dict = {"number": 0, "message": ""}
>>> process_hal = Process(**process_dict)
>>> process_hal
Process(links=None, embedded={'message': ''}, number=0)

[jtc42]

@ELC are you happy for this one to be merged?